1. Field of the Invention
This invention relates to new drilling fluids based on ester oils and to invert drilling muds based thereon which combine high ecological compatibility with good stability and performance properties. One important application for the new drilling fluids is in offshore drilling for the development of oil or gas sources, the particular object of the invention in this regard being to provide technically useful drilling fluids of high ecological compatibility. The use of the new drilling fluids is of particular importance in, but is not limited to, the offshore sector. The new drilling fluids may also be used quite generally for land-supported drilling, including for example geothermal drilling, water drilling, geoscientific drilling and mine drilling. In this case, too, the ester-based drilling fluids selected in accordance with the invention basically simplify ecotoxic problems to a considerable extent.
2. Discussion of Related Art
It is known that liquid drilling fluids for sinking bores in rock and bringing up the rock cuttings are slightly thickened, water-based or oil-based fluid systems. Oil-based systems are being increasingly used in practice, particularly in offshore drilling or in the penetration of water-sensitive layers.
Oil-based drilling fluids are generally used in the form of so-called invert emulsion muds which consist of a three-phase system, namely: oil, water and finely divided solids. The emulsions in question are of the water-in-oil (w/o) emulsion type, i.e. the aqueous phase is present in the continuous oil phase in heterogeneous fine dispersion. There are a whole range of additives, including in particular emulsifiers and emulsifier systems, weighting agents, fluid loss additives, alkali reserves, viscosity regulators and the like, for stabilizing the system as a whole and for establishing the desired performance properties. Full particulars can be found, for example, in the Article by P. A Boyd et al entitled "New Base Oil Used in Low-Toxicity Oil Muds" in Journal of Petroleum Technology, 1985, 137 to 142 and in the Article by R. B. Bennet entitled "New Drilling Fluid Technology--Mineral Oil Mud" in Journal of Petroleum Technology, 1984, 975 to 981 and the literature cited therein.
Oil-based drilling muds were originally made from diesel oil fractions containing aromatic constituents. For the purposes of detoxification and reducing the ecological problems thus created, it was then proposed to use hydrocarbon fractions substantially free from aromatic compounds, now also known as "nonpolluting oils", as the continuous oil phase, cf. the literature cited above. Although certain advances were achieved in this way through elimination of the aromatic compounds, a further reduction in the environmental problems caused by drilling fluids of the type herein seems to be urgently required. This applies in particular to the sinking of offshore wells for the development of oil and gas sources because the marine ecosystem is particularly sensitive to the introduction of toxic and non-readily degradable substances.
The relevant technology has for some time recognized the significance of ester-based oil phases for solving these problems. Thus, U.S. Pat. Nos. 4,374,737 and 4,481,121 describe oil-based drilling muds in which nonpolluting oils are said to be used. Non-aromatic mineral oil fractions and vegetable oils of the peanut oil, soybean oil, linseed oil, corn oil and rice oil type, and even oils of animal origin, such as whale oil, are mentioned alongside one another as nonpolluting oils of equivalent rank. The ester oils of vegetable and animal origin mentioned here are all triglycerides of natural fatty acids which are known to be environmentally safe and which, ecologically, are distinctly superior to hydrocarbon fractions, even where they have been de-aromaticized.
Interestingly, however, not one of the Examples in the U.S. patents cited above mentions the use of such natural ester oils in invert drilling muds of the type herein. Mineral oil fractions are used throughout as the continuous oil phase.
The investigations on which the present invention is based have shown that the use of readily degradable oils of vegetable or animal origin, which was considered in the prior art, is not feasible for practical reasons. The rheological properties of such oil phases cannot be controlled for the wide temperature range required in practice of 0.degree. to 5.degree. C. on the one hand, up to 250.degree. C. and higher on the other hand.
Ester oils of the type in question here do not in fact show the same in-use behavior as the pure hydrocarbon-based mineral oil fractions used hitherto. In practice, ester oils irrespective of their constitution undergo partial hydrolysis precisely in the w/o invert drilling muds, resulting in the formation of free carboxylic acids. These free carboxylic acids in turn react with the alkaline constituents always present in the drilling mud systems of the type in question here, for example with the alkali reserve used for corrosion prevention, to form the corresponding salts. However, salts of highly hydrophilic bases and the acids having carbon chain lengths of about C12 to C24 predominantly encountered in oils of natural origin are known to be compounds having comparatively high HLB values which, in particular, lead to the formation and stabilization of o/w emulsions. Use is made of this on a very wide scale in the field of detergents and cleaning preparations. However, the formation of even limited quantities of such o/w emulsifier systems must interfere with the w/o emulsions required for solving the problem addressed by the invention and, hence, must lead to problems.
Earlier applications U.S. Ser. No. 07/452,457 filed Dec. 18, 1989 titled "The use of selected ester oils in drilling muds (I)" and U.S. Ser. No. 07/452,988 filed Dec. 19, 1989 titled "The use of selected ester oils in drilling muds (II)") relate to the use of ester oils based on selected monocarboxylic acids or monocarboxylic acid mixtures and monofunctional alcohols containing at least 2 and preferably at least 4 carbon atoms. The earlier applications show that it is possible with the disclosed esters or ester mixtures of monofunctional reactants not only to establish satisfactory rheological properties in the fresh drilling mud, but also to work with selected known alkali reserves in the drilling mud and thus to prevent unwanted corrosion. The alkali reserve is formed by the addition of lime (calcium hydroxide) or by the presence of zinc oxide or comparable zinc compounds. However, an additional limitation is appropriate in this regard. If unwanted thickening of the oil-based invert drilling mud is to be prevented in practice, the quantity of alkalizing additive and, in particular, the quantity of lime have to be limited. According to the disclosure of the earlier applications mentioned, the maximum addition envisaged is put at about 2 lb/bbl (barrel) oil mud.
In these earlier applications, the alcohol component used in the ester oil is consciously limited to at least 2 and preferably to at least 4 carbon atoms. The reason for this is the fact well known among experts in the field in question that, in the presence of water, the methylesters of carboxylic acids are hydrolyzed particularly easily with formation of the free carboxylic acids. With higher alcohol components, this tendency towards hydrolysis is weakened. The use of monocarboxylic acid methylesters in invert drilling muds of the w/o emulsion type appeared pointless. Their pronounced tendency towards hydrolysis and the resulting premature formation of relatively large quantities of the free carboxylic acids or the salts with o/w emulsifier properties formed therefrom had been expected to result in lasting damage to the system as a whole.